In most steam boilers, the steam temperature is controlled at the boiler outlet by injecting water in the mostly subcooled state at one or more points in the superheater train. This water is usually removed before the economizer and evaporates after the injection point, so that the enthalpy and therefore the temperature of the superheated steam is lowered at the injection or mixing point of the respective heating surfaces. This also reduces the temperature at the outlet of the respective heater surface.
In known methods, the temperature reduction is limited by the water injection, and only such an amount of water can be injected that the steam remains sufficiently superheated after the mixing of the injection water and steam. The purpose is the prevention of temperature shocks that may be created by impermissibly large temperature drops, or caused by the impact of cold injection water droplets on the hot, steam-carrying pipes.
Known injection control loops are constructed in a cascading manner, whereby the subcontrol loop controls the inlet temperature before the respective superheater. The information of the inlet temperature is lost, however, as soon as a two-phase mixture at saturation temperature is present after mixing. The controlling then would have to be performed manually.
As a result, the possible drop in temperature through the injection of water in the designs known so far is critically limited, since it is not possible to inject an amount of water that would lower the steam temperature to the saturated steam temperature.
The known injection coolers furthermore have a very high constructive expenditure; each injection cooler between the heating surfaces shares the heating surface bundles and requires corresponding headers and connecting pipes.
In respect to energy, the past practice of injecting water between or after the superheater heating surfaces has the disadvantage that the steam temperature control on the one hand is coupled with a drop of the already achieved high steam temperature with, on the other hand, a water evaporation at a low temperature level. The result is an adverse effect on the efficiency of the superheater or boiler. Another disadvantage is, in respect to materials, the heating surface design for higher temperatures.
Of special significance is the problem of steam temperature control, e.g., in refuse incineration plants, i.e., in plants with increasing fouling of the heating surfaces in the combustion chamber. This results in a steady increase of the flue gas temperature before the superheater. In order to be able to maintain the steam temperature after the superheater, the injection water quantity must be increased. This often requires installation of several intermediate injection coolers.